1. Field of the Invention
The present invention relates to a heating device which can be utilized for heating an unfixed image in a fixing device which is arranged in an electrophotographic image forming apparatus such as a copying machine, printer or the like, and fixes the unfixed image such as a toner image onto a record member bearing the unfixed image.
2. Description of Related Art
A fixing device in an image forming apparatus such as a printer or a copying machine generally includes a heating device provided with a heating roller. A record member bearing an unfixed image such as a toner image is moved between the heating roller and a backup member (generally, a pressure roller) opposed thereto, so that the unfixed image is heated and pressed to be fixed onto the record member.
In many cases, the heating roller includes a heater such as a halogen lamp heater, and the roller is heated by heat radiated from the heater.
In the heating device including the heating roller which contains the heater such as a halogen lamp heater as a heat source, however, the heater cannot rapidly heat a surface of the heating roller to a predetermined fixing temperature after start of power supply to the heater so that a long preheating time (i.e., a warming-up time) is required before the heating roller reaches the predetermined temperature after power-on of the image forming apparatus. This prevents easy operation of the apparatus.
For reducing the preheating time, a heating roller has been proposed. This roller includes a core roller and a resistance heating member formed on the core roller. The resistance heating member is made of a substance which generates heat when an electric current flows therethrough. The heating roller of this type has a good electrothermal converting efficiency, and can rapidly raise the surface temperature of the heating roller to the predetermined temperature after start of current supply to the resistance heating member so that the preheating time of the heating roller can be reduced.
For supplying a current to the resistance heating member, the heating device having the heating roller of the above type is generally provided with a current receiver member which is electrically connected to the resistance heating member and rotates together with the heating roller, and a current supply member which is in contact with the current receiver member. The resistance heating member is supplied with the current through these supply and receiver members. When the heating roller rotates, the current receiver member also rotates together with the heating roller, and slides on the current supply member.
In the heating device in which the current is supplied to the resistance heating member through the supply and receiver members in sliding contact with each other, when an electrical contact resistance at the mutual contact surfaces of the supply and receiver members increases, a large voltage drop occurs at the mutual contact surfaces, and a voltage across the opposite ends of the resistance heating member lowers. The contact resistance at the mutual contact surfaces increases if the surface roughness(es) at the mutual contact surface(s) of the supply and/or receiver members are large and therefore the area of the mutual contact region between these members is small.
In view of the above, such a structure has been proposed that one of the current supply member and the current receiver member is made of a material containing carbon (carbon material) such as material containing carbon and copper (carbon material containing copper), and the other is made of metal or the like harder than the carbon material. Usually, the current supply member is made of the carbon material in view of maintainability. If the current supply member is made of carbon material, a carbon film or layer is formed at the surface of the current receiver member when the current receiver member slides on the current supply member in accordance with rotation of the heating roller. This carbon film smoothens the mutual contact surfaces of the supply and receiver members, which reduces the electrical contact resistance at the mutual contact surfaces. Accordingly, the current can be supplied more stably to the resistance heating member through the supply and receiver members.
The carbon film is formed as a result of shaving of the current supply member made of carbon material due to sliding between the supply and receiver members. The shaving of the current supply member is affected by the surface roughnesses of the contact surfaces of the current supply member and current receiver member as well as the difference between these surface roughnesses. However, the conventional heating devices have been designed without particularly taking the above fact into consideration.
If the current supply member is made of a carbon material containing copper as described above, the content of copper affects the electrical contact resistance at the mutual contact surfaces of the receiver and supply members as well as a specific resistance of the current supply member itself. Depending on the content of copper, therefore, the voltage across the opposite ends of the resistance heating member may excessively lower, and the time required for temperature rising (the preheating time) of the heating roller may excessively increase. Further, the content of copper in the carbon current supply member affects the sliding property at the mutual contact surfaces of the receiver and supply members as well as thicknesses or quantities of wear of the supply and receiver members.
In the prior art, however, the content of copper in the current supply member made of carbon material containing copper is not particularly taken into consideration.
The following problem also arise when the current supply member made of carbon material is employed. In an early stage before the carbon film is sufficiently formed at the contact surface of the current receiver member in contact with the current supply member as a result of sliding between the supply and receiver members, the electrical contact resistance at the mutual contact surfaces of the supply and receiver members changes significantly and therefore the voltage cannot be stably supplied to the resistance heating member. In other words, the above problem occurs in such an early stage that the carbon film is growing on the surface of the current receiver member not covered with the carbon film.